1. Field of the Invention
The present invention relates to liquid crystal display devices, in particular, to a liquid crystal display device including separate functional members, such as a touch panel, provided in front of a liquid crystal module.
2. Description of the Related Art
There has been widely used a recent liquid crystal display device as a display section in variety of products. Therefore, a separate functional member such as a touch panel is sometimes provided on the front surface side of a liquid crystal module (display surface) in a liquid crystal display device. Alternatively, a transparent protection plate is sometimes provided on the front surface side of a liquid crystal module (display surface) as a functional member to protect a polarization plate in the liquid crystal module and/or to protect a glass substrate from damage caused by externally applied impacts.
A functional member such as a touch panel or a protection plate is arranged so that a space is left between a functional member and a liquid crystal module. There is a layer of air in the space between the liquid crystal module and the functional member. The layer of air causes externally entered light to be reflected from an interface between the layer of air and the back surface of the functional member or to be reflected from an interface between the layer of air and the front surface of the liquid crystal module. This leads to a problem that the display contents in the liquid crystal module are difficult to view.
Japanese Examined Utility Model (Registration) Application Publication No. 06-24812 discloses an arrangement that prevents the reflection problem caused by a liquid crystal display device in which a transparent protection plate is provided in front of a liquid crystal module. The following description deals with an arrangement of a liquid crystal display device disclosed in the Japanese Examined Utility Model (Registration) Application Publication No. 06-24812 with reference to FIG. 2.
In the liquid crystal display device illustrated in FIG. 2, a protection panel 110 including a protection plate 111 is provided in front of a liquid crystal module 100. There is a space left between the liquid crystal module 100 and the protection panel 110, and a layer of air is secured in the space.
For the protection panel 100, an anti-reflection film 112 is provided on the front surface side of the protection plate 111, and a polarization plate 113 and a λ/4 plate 114 are provided on the back surface side of the protection plate 111 in the order of being the closer side to the protection plate 111.
In the liquid crystal module 100, a liquid crystal layer is sandwiched between two transparent substrates, and an alignment film is provided on a surface of each of the substrates with which surface the liquid crystal layer makes contact. In FIG. 2, the liquid crystal module 100 is illustrated in a simplified manner. Namely, the transparent substrates, the alignment films, and the liquid crystal layer are not illustrated in FIG. 2. A λ/4 plate 101 is provided on the front surface side of the liquid crystal module 100, and a polarization plate 102 is provided on the back surface side of the liquid crystal module 100.
The anti-reflection film 112 prevents light that has externally entered from the front surface side of the liquid crystal display device from being reflected from the front surface of a protection plate 101.
The polarization plate 113 and the λ/4 plate 114 avoid that the light externally entered from the front surface side of the liquid crystal display device is reflected from an interface between the back surface of the protection panel 110 and the layer of air, or from an interface between the front surface of the liquid crystal module 100 and the layer of air. This makes it possible to prevent the reflection from being viewed by a viewer. In the liquid crystal display device illustrated in FIG. 2, one of the polarization plates used for display control (the polarization plate provided on the front surface side of the liquid crystal layer) is provided on the front surface side of the layer of air (i.e., on the side of the protection panel 100), and the one of the polarization plates is combined with a λ/4 plate. This combination realizes a circularly polarized light means. With this arrangement, the light externally entered from the front surface side of the liquid crystal display device becomes linear polarized light while first passing though the polarization plate 113. The linearly polarized light is reflected from the interface between the back surface of the protection panel 110 and the layer of air or is reflected from the interface between the front surface of the liquid crystal module 100 and the layer of air. Then, the reflected light passes through the λ/4 plate 114 twice before reaching the polarization plate 113, thereby resulting in its plane of polarization being rotated by 90°. This causes the reflected light to be blocked by the polarization plate 133.
In addition, the λ/4 plate 101 on the side of the liquid crystal module 100 is provided to aid the display control (tone control) function of the polarization plate 113. In the arrangement illustrated in FIG. 2, the polarization plate 113 is the one of the two polarization plates for display control, which polarization plate is provided on the side of the protection panel 110. In order for the polarization plate 113 to carry out a display control function, the outgoing display light from the liquid crystal module 100 should be linearly polarized light during passing through the polarization plate 113. Without the λ/4 plate 101, however, outgoing display light from the liquid crystal module 100 is linearly polarized light after it passes through the polarization plate 102. But the linearly polarized light is converted into circularly polarized light while passing through the λ/4 plate 114, thereby resulting in that the circularly polarized light enters into the polarization plate 113.
On the contrary, with the λ/4 plate 101, the outgoing display light from the liquid crystal module 100 is converted into circularly polarized light while passing through the λ/4 plate 101 and is then converted into linearly polarized light by the λ/4 plate 114. As a result, the linearly polarized display light thus converted is entered into the polarization plate 113 from the liquid crystal module 100. This allows the polarization plate 113 to have a display control function.
As a conventional example that prevents externally entered light from being reflected, Japanese Unexamined Patent Application Publication No. 2000-321558 discloses a liquid crystal display device that has a touch panel in front of a liquid crystal module. The following description deals with an arrangement of the liquid crystal display device disclosed in the Japanese Unexamined Patent Application Publication No. 2000-321558 with reference to FIG. 3.
In the liquid crystal display device illustrated in FIG. 3, a touch panel section 130 including a touch panel 131 is provided in front of a liquid crystal module 120. FIG. 3 does not illustrate the details of the touch panel 131 (members such as substrates and electrodes). There is a space left between the liquid crystal module 120 and the touch panel section 130, so that a layer of air is present in the space.
In the touch panel section 130, a polarization plate 132 and a λ/4 plate 133 are provided in this order, i.e., in the order of being closer to the front surface of the touch panel section 130. An anti-reflection film 135 and a λ/4 plate 134 are provided in this order, i.e., in the order of being closer to the back surface of the touch panel section 130.
In the liquid crystal module 120, a liquid crystal layer is sandwiched between two transparent substrates, and an alignment film is provided on a surface of each of the substrates with which surface the liquid crystal layer makes contact. In FIG. 3, the liquid crystal module 120 is illustrated in a simplified manner. Namely, the transparent substrates, the alignment films, and the liquid crystal layer are not illustrated in FIG. 3. A polarization plate 121 is provided on the front surface side of the liquid crystal module 120, and a polarization plate 122 is provided on the back surface side of the liquid crystal module 120.
The polarization plate 132 and the λ/4 plate 133 in the touch panel section 130 prevents external light that has entered from the front surface side of the liquid crystal display device from being reflected from the inside of the touch panel 131. As compared to a mere protection plate, the light tends to be reflected from the inside of the touch panel 131 because the touch panel 131 includes electrodes and the layer of air. Therefore, the polarization plate 132 and the λ/4 plate 133, which have stronger anti-reflective capability than a mere anti-reflection film, are provided on the front surface side of the touch panel section 130.
Accordingly, the reflected light occurring inside the touch panel 131 passes through the λ/4 plate 133 twice for a time period from the time when the reflected light first passes through the polarization plate 132 to the time when the reflected light is reflected from the inside of the touch panel 131 so as to reach the polarization plate 132. This causes the reflected light to be blocked off by the polarization plate 132.
Also, the λ/4 plate 134 in the touch panel section 130 is provided so that almost all the display light from the liquid crystal module 120 can transmit the polarization plate 132. An anti-reflection film 135 in the touch panel section 130 is arranged to prevent external light that has entered from being reflected from an interface between the back surface of the touch panel section 130 and the layer of air.
Unfortunately, however, the liquid crystal display device in the Japanese Examined Utility Model (Registration) Application Publication No. 06-24812 has a problem of decline in visual characteristics. In the arrangement illustrated in FIG. 2, the polarization plate 113 for display control is provided on the side of the protection panel 110. The two λ/4 plates 114 and 101 are provided between the polarization plate 113 and the liquid crystal module 100. In this case, the visual characteristics of the liquid crystal display device including the liquid crystal module 100 and the protection panel 110 are also affected by the visual characteristics of the λ/4 plates 114 and 101. As a result, the visual characteristics (the contrast viewed from an oblique angle in particular) of liquid crystal display device are declined.
However, the liquid crystal display device in the Japanese Unexamined Patent Application Publication No. 2000-321558 includes two polarization plates 121 and 122 for display control in the liquid crystal module 120, thereby preventing the decline in the visual characteristics. This is because the visual characteristics of liquid crystal display device are not affected by the visual characteristic of the λ/4 plate.
In the arrangement of the liquid crystal display device disclosed in the Japanese Unexamined Patent Application Publication No. 2000-321558, the polarization plate 132 and the λ/4 plate 133 are provided for anti-reflection of externally entered light. Without the additional arrangement of the λ/4 plate 134, however, the polarization plate 132 absorbs the display light, thereby causing a decline in the display brightness.
According to the arrangement of FIG. 3, in the liquid crystal module 120, the display light is linearly polarized light that has been subjected to a display control when the display light passes through the polarization plate 121. The following description deals with a case where the touch panel section 130 does not have a λ/4 plate 134.
In order for the polarization plate 132 and the λ/4 plate 133 to have an anti-reflective function, the angle between a lag axis of the λ/4 plate 133 and an absorption axis of the polarization plate 132 is set to 45°. When the angle between a transmission axis of the polarization plate 121 in the liquid crystal module 120 and the lag axis of the λ/4 plate 133 is 45°, the display light from the liquid crystal module 120 becomes circularly polarized light while passing through the λ/4 plate 133. This causes about half amount of the display light to be further blocked off while passing through the polarization plate 132.
When the transmission axis of the polarization plate 121 and the lag axis of the λ/4 plate 133 are parallel (or orthogonal) to each other, the display light from the liquid crystal module 120 does not change its polarization state during passing through the λ/4 plate 133. In this case, however, the display light enters into the polarization plate 132 as the linearly polarized light which inclines at an angle of 45° with respect to the absorption axis of the polarization plate 132. In this case, about half of the display light is blocked off by the polarization plate 132, too.
On the contrary, in the arrangement having the λ/4 plate 134, when the outgoing display light from the liquid crystal module 120 passes through the λ/4 plate 134, the outgoing display light is converted from linearly polarized light to circularly polarized light. When the display light thus converted passes through the λ/4 plate 133, the display light is further converted from circularly polarized light to linearly polarized light. Furthermore, the linearly polarized light is entered into the polarization plate 132 as the linearly polarized light which travels in parallel to the transmission axis of the polarization plate 132. This avoids a situation in which the polarization plate 132 absorbs the display light.
In case of an arrangement in which the λ/4 plate 133 and the λ/4 plate 134 are provided, however, the problem caused this time is, without an anti-reflection film 135, it is not possible to avoid the reflection from an interface between the touch panel section 130 and the layer of air.
Namely, in cases where the λ/4 plate 133 and the λ/4 plate 134 are provided in the touch panel section 130, externally entered light, reflected from an interface between the touch panel section 130 and the layer of air, (i) first passes through the polarization plate 132, (ii) passes through the λ/4 plate 133 and 134, respectively twice, and then (iii) reaches again the polarization plate 132. This causes the externally entered light to have a plane of polarization rotated by 180° and then to reach the polarization plate 132, after the externally entered light passes through the polarization plate 132. As a result, the externally entered light thus reached (the reflected light) can not be blocked by the polarization plate 132.
Therefore, the liquid crystal display device illustrated in FIG. 3 requires the anti-reflection film 135 to block the reflection from the interface between the touch panel section 130 and the layer of air. This makes the process steps complicated and increases the cost, which problems are derived from manufacturing of the anti-reflection film 135.
Moreover, in order to avoid the reflection from an interface between the liquid crystal module 120 and the layer of air, it is in fact necessary to further provide an anti-reflection film on the front surface side of the liquid crystal module 120. This is not explicitly described in Japanese Unexamined Patent Application Publication No. 2000-321558 but causes further problems of making the steps complicated and increasing the cost.